A mirror is a reflective surface that is smooth enough to be able to form an image. The best known
example is the plane mirror that most people have at home. In it, a parallel beam of light changes its direction as a whole, whilst still remaining parallel; the images formed by a plane mirror are virtual images, of the same size as the original object (see mirror image). There are also parabolic concave mirrors, where a parallel beam of light becomes a convergent beam, whose rays intersect in the focus of the mirror. Finally, there are convex mirrors, where a parallel beam becomes divergent, with the apparent intersection occurring behind the mirror. Note that spherical concave and convex mirrors do not have a single focal point, as often erroneously described in high school physics text books.

Early mirrors consisted of a plate or sheet of polished metal, often silver when the reflected image was for viewing (such as for personal grooming) but also of other metals when only the intensity of reflected light was important.

Modern mirrors usually consist of a thin layer of aluminium (or sometimes other metals) deposited on a sheet of glass. They are usually back silvered, where the reflecting surface is viewed through the glass sheet; this makes the mirror durable, but lowers the image quality of the mirror due to extraneous reflections from the front surface of the glass. This type of mirror reflects about 80% of the incident light. Front silvered mirrors, where the reflecting surface is placed on the front surface of the glass, have a better image quality but are easily scratched and damaged. They reflect 90% to 95% of the incident light. Astronomical mirrors are of the
latter type, and they have to be resurfaced every now and then
to keep their quality.

For scientific optical work, dielectric mirrors are often used. These are glass (or sometimes other material) substrates on which one or more layers of dielectric material are deposited, to form an optical coating. By careful choice of the type and thickness of the dielectric layers, the range of wavelengths and amount of light reflected from the mirror can be specified. The best mirrors of this type can reflect >99.999% of the light (in a narrow range of wavelengths) which is incident on the mirror.

A beam of light reflects off of a mirror at an angle of reflection that is equal to its angle of incidence. That is, if the beam of light is shining on a mirror's surface at a 30° angle from vertical, then it reflects from the point of incidence at a 30° angle from vertical in the opposite direction.

There exist rear view sunglasses, of which the left end of the left glass and the right end of the right glass work as mirrors.

A one-way mirror, also called two-way mirror (!), reflects about half of the light and lets the other half pass. It is a sheet of glass coated with a layer of metal only a few dozen atoms thick, allowing some of the light through the surface. It is used between a dark room and a brightly lit room. From the dark side it looks like a transparent window and from the brightly lit side like a mirror. It may be used to observe criminal suspects, customers (to watch out for theft), etc. The same type of mirror, when used in an optical instrument, is called a half-silvered mirror. Its purpose is to split a beam of light so that half passes straight through, while the other half is reflected -- this is useful for interferometry.

A decorative reflecting sphere of thin metal-coated glass, working as a reducing wide-angle mirror, is sold in the period before Christmas, to be used as Christmas tree decoration called a bauble.